Method and system for non-destructive dye penetration testing of a surface

ABSTRACT

A method is presented for non-destructive testing of the state of a surface that may have cracks in it by observing waves emitted by a dye applied to the surface and present in the cracks in response to an incident excitation beam of wavelength appropriate to the dye. The incident excitation beam is an ultraviolet light. The beam is made of rectilinear polarized waves. Waves emitted by the dye are observed through a rotatable polarized wave analyzer. The analyzer is rotated first to eliminate from observation the wave due to the residual dye on the surface and thereafter to determine the depths of the cracks.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to techniques for non-destructive dye penetrationtesting of the state of a surface, such as the dye penetration andmagnetoscopic techniques in particular.

Non-destructive dye penetration testing, which is used in particular forchecking for the presence of cracks in a surface, entails theapplication to that surface of a dye under conditions enabling the dyeto penetrate into the defects to be detected, illuminating the surfacewith incident light, and observing the light emitted by the dye presentin the cracks and by the residual dye on the surface.

The surface is illuminated by a beam from a mercury vapor lamp, forexample, or from a neon tube, and which includes radiation capable ofexciting the dye, which responds by emitting visible monochromatic lightthat can be observed by means of a photosensitive system, for example,possibly associated with means for producing a digital image that isthen processed by an image processing system.

The color of the visible light depends on the dye used. It is orange,for example, if the dye is rhodamine 6G.

2. Description of Related Art

A technique of the above kind is described in the publication FR 2 711426, for example.

The problem arises of optimizing the observation, in particularoptimizing the observation of cracks, which is disturbed by thesimultaneous observation of an image of residual traces of dye whichremain on the surface.

One proposal for optimizing the observation involves processing thedigital image, for example as described in the aforementionedpublication.

Another problem is determining the depth of the cracks.

The publication FR 2 736 152 describes a method and a dye penetrationsystem for determining the dimensions of defects.

The above two publications indicate the difficulty and the consequentialcomplexity of methods and systems that have been designed to optimizethe images and to assess the dimensions of cracks.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a method and a system which areremarkably simple, which optimize the images and which assess the depthof the cracks.

The invention is based on the observation that when the dye is excitedby a rectilinear polarized wave the dye in an area of the surface or ina crack emits a rectilinear polarized wave at an angle to the incidentrectilinear polarized wave which depends on the thickness of the productin the area or the crack concerned.

The invention uses the angle between the incident rectilinear polarizedwave and the rectilinear polarized wave emitted by the dye to eliminatefrom the observation areas where residual dye is present on the surfaceand retain only areas in which the dye has penetrated into the cracks.This approach is based on the fact that the angle corresponding to awave reflected by residual dye on the surface is different from thatcorresponding to the dye present in a crack because the thickness of theresidual dye on the surface is always less than the thickness of dye ina crack.

According to another aspect of the invention, the angles correspondingto the cracks are used to determine the depths of the cracks.

A system for implementing the invention therefore includes means forproducing a rectilinear polarized incident wave at a wavelength chosento excite the dye, transmission means for guiding the wave toward thesurface to be studied, observation means for observing rectilinearpolarized waves emitted by dye on the surface and in the cracks, andmeans on the path of the emitted waves, between the surface and theobservation means, for selecting emitted waves according to the anglebetween the incident rectilinear polarized wave and the rectilinearpolarized wave emitted by the dye.

The surface under examination is illuminated with a rectilinearpolarized wave from a non-polarized light source associated with apolarizer or preferably from a polarized light source.

This applies in particular to a laser, which delivers a rectilinearpolarized wave with parallel edges.

The laser has the further advantage of emitting perfectly monochromaticlight in a very fine beam which is coherent over great distances.

Rectilinear polarized waves emitted by the dye are selected by one ormore polarized wave analyzers on the path of the waves.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are described hereinafter with reference tothe figures of the accompanying drawings, in which:

FIG. 1 is a diagram of a first embodiment of a system according to theinvention applied to a dye penetration technique for observing cracks ina surface;

FIG. 2 is a diagram of a second embodiment using a dye penetrationtechnique for simultaneously observing two opposite faces of a partwhich can rotate and move along X, Y and Z axes;

FIG. 3 is a diagram of a third embodiment using a dye penetrationtechnique for simultaneously observing two opposite faces of a plate andincluding a set of oscillating mirrors which are controlled to scan theincident beam over the faces observed;

FIG. 4 is a diagram of a fourth embodiment using a dye penetrationtechnique for examining bores or cells; and

FIG. 5 is a diagram of a fifth embodiment applied to the magnetoscopictesting of bars.

DETAILED DESCRIPTION OF THE INVENTION

The systems implementing the invention shown in the figures include:

a laser (1),

transmission means (T) for transmitting the beam from the laser to asurface (S) to be studied or to a calibration surface, and

an observation video camera (2) equipped with a telephoto lens and ananalyzer (3).

The laser (1) preferably emits polarized ultraviolet light centered on awavelength appropriate to the dye used, for example a wavelength of 330nanometers.

The transmission (T) means can be of highly varied types:

in the FIG. 1 embodiment, the beam from the laser (1) is transmitted byoptical fibers (4), the transmission means (T), to the surface (S) to beexamined,

in the FIG. 2 embodiment a plate (5) at the exit of the laser (1) splitsthe laser beam into a portion (6) which is conveyed by optical fibers orotherwise to a surface (7) of a part (8) to be observed and a portion(9) which is conveyed by optical fibers or otherwise to an oppositesurface (10) of the part (8),

in the FIG. 3 embodiment a plate (5) at the exit from the laser (1), asin the FIG. 2 embodiment, splits the beam from the laser into two beams(6, 9) which illuminate two respective faces (7) and (10) of a wall (8);in this example the system includes two sets of mirrors (M) and (MM) fortransmitting the two beams to the surfaces (7, 10),

in the FIG. 4 embodiment, the beam from the laser (1) is transmitted byoptical fibers (4) to an endoscope (11) which enables the beam to reachareas to which access is difficult, for example to illuminate theinternal surface of a bore or a cell, and

in the FIG. 5 embodiment, the beam from the laser (1) passes through aplate (12) and is then conveyed to the surface to be examined, in thisinstance that of a bar (13), by optical fibers (14).

The above examples do not exclude other transmission means.

The incident laser beam is much narrower than the beam from a mercuryvapor lamp or a neon tube. The invention therefore provides means forscanning the beam over the surface to be examined, either by moving thebeam or by moving the surface.

In the FIG. 2 embodiment, the part (8) to be examined is placed on aturntable (15) which can be moved in translation along an axis (X) andwhich can move up and down along an axis (Z) parallel to the rotationaxis of the part.

In the FIG. 3 embodiment, the mirrors for reflecting the two portions ofthe beams from the laser comprise, firstly, fixed mirrors (M1, M2) and,secondly, mirrors (MM1, MM2, MM3 and MM4) which can oscillate under thecontrol of a computer programmed so that the beams scan the surfaces tobe examined.

In the FIG. 4 embodiment, the endoscope includes a support 16 which canrotate about a rotation axis (R), a telescopic waveguide (17) along therotation axis (Z) and an end prism (18) which can move along the (X, Y)axes, and the part (P) which includes the bores (19) to be examined isplaced on a turntable (20). The line (21) in the figure symbolizes thefact that the end prism (18) can be lowered into the bore (19).

In the FIG. 5 embodiment, the part (13) observed is a bar which canrotate and the end of the fiber optic guide (14) is carried by acarriage (22) which can move parallel to the bar.

The observation video camera (2) and its polarized wave analyzer (3) areduplicated if it is necessary to observe two surfaces simultaneously, asis the case in FIGS. 2 and 3 embodiments.

Splitter plates are used, if required, to pass the incident rays anddeflect the emergent rays (FIGS. 3 and 5).

In the FIG. 4 embodiment, an optical fiber coupler (23) is used toenable the same bundle of optical fibers (4) to transmit the incidentbeam from the laser (1) to the part (P) and to transmit the wave emittedby the excited dye to the video camera (2).

This is possible because the video camera used is not sensitive toultraviolet radiation.

The equipment is calibrated using a block with cracks of known depththat has been prepared by dye penetration or magnetoscopy.

In the absence of the analyzer (3), or by locking the analyzer to thepolarity of the laser (1), the image observed shows the presence ofcracks in the block buried in light spots with the same wavelengthcaused by residual dye on the surface of the block.

If the analyzer according to the invention is rotated, the unwantedspots are seen to disappear progressively, leaving only the images dueto the calibration cracks for one particular angle of rotation of theanalyzer.

The analyzer is rotated further to examine the successive images ofcracks in increasing depth order: it is therefore possible to draw up acalibration curve relating the angle of rotation of the analyzer to thedepth of the cracks, enabling the depths of cracks in a surface examinedto be determined subsequently.

A double analyzer is preferably used, i.e. an analyzer which includestwo successive analyzers (3 a, 3 b), namely a front analyzer (3 a)eliminating unwanted spots and a rear analyzer (3 b) determining thedepth of the cracks.

The two analyzers are mounted in a common turret, for example, and arerotated at the same time until the unwanted spots are eliminated, afterwhich only one of the analyzers is rotated, to evaluate the depths ofthe cracks.

Each analyzer is of a type known in the art, for example a plate on oneface of which are parallel lines of identical prisms.

The invention is not limited to the above embodiments.

What is claimed is:
 1. A method of non-destructive testing of the stateof a surface which may have cracks in it by observing waves emitted by adye applied to the surface and present in the cracks in response to anincident excitation beam of wavelength appropriate to the dye, whereinthe incident excitation beam is an ultraviolet light, wherein said beamis made of rectilinear polarized waves, wherein waves emitted by the dyeare observed through a rotatable polarized wave analyzer, and whereinsaid analyzer is rotated first to eliminate from observation the wavedue to the residual dye on the surface and thereafter to determine thedepths of the cracks.
 2. A method according to claim 1 wherein the saidincident excitation beam is centered on a wavelength of 330 nanometers.3. A method according to claim 1 or 2 wherein the waves emitted by thedye are observed by a camera which is not sensitive to ultravioletradiation.
 4. A method according to claim 1 or 2, which comprises acalibration process to establish angles of rotation of the analyzerwhich successively eliminate from observation cracks of increasingdepth.
 5. A method according to claim 1 or 2, wherein said surface isscanned with the incident beam by moving the surface or the incidentbeam during observation.
 6. A system for implementing a dye penetrationor magnetoscopic method using a dye applied to the surface and presentin the cracks of a surface to be examined, which system includes:production means for producing a polarized ultraviolet beam,transmission means for guiding that beam toward the surface to beexamined, observation means of observing the waves emitted by the dyewhen excited by said beam, wherein said production means are selected toprovide a rectilinear polarized ultraviolet beam, and wherein the systemcomprises a rotatable polarized wave analyzer on the path of the wavesemitted by the dye between the surface and the observation means,whereby a rotation of the analyzer allows first to eliminateprogressively from the observation the waves due to the residual dye onthe surface and thereafter to determine the depth of the cracks.
 7. Asystem according to claim 6, wherein the production means comprise alaser.
 8. A system according to claim 6 or 7, wherein the ultravioletbeam is centered on a wavelength of 330 nanometers.
 9. A systemaccording to claim 6, wherein said observation means is a camera whichis not sensitive to ultraviolet radiation.
 10. A system according toclaim 6 or 9, including means for moving the surface or the incidentbeam to scan the surface with the incident beam during observation. 11.A system according to claim 6 or 9, including oscillating mirrorscontrolled by computer to deflect the incident beam to cause it to scanthe surface.
 12. A device according to claim 6 or 9, wherein thetransmission means include an endoscope terminating in a prism forentering a bore or a cell of a piece to be examined.
 13. A systemaccording to claim 6 or 9, wherein the angles of rotation of theanalyzer have been calibrated corresponding to the depths of cracks. 14.A system according to claim 6 or 9, including two successive rotatableanalyzers respectively for eliminating unwanted spots due to residualdye on the surface and for determining the depth of cracks.